This invention relates to electromagnetic projectile launchers and more particularly to such launchers wherein the means for conducting current between a pair of generally parallel conductive rails includes a conductive element and a plasma.
Electromagnetic projectile launchers are known which comprise of pair of generally parallel conductor rails, a sliding conductive armature between the rails, a source of high current, and a switch for commutating this current into the rails and through the armature. The passage of electrical current through the rails and the armature produces an electromagnetic force on the armature which propels it along the conductive rails. Sliding metallic armatures have generally been used in launchers which accelerate projectiles to velocities below 1000 meters per second. Launchers which utilize metallic armatures have experienced considerable rail damage caused by the sliding armature, particularly where high armature velocities are involved. For smaller projectiles and higher velocities, a plasma or arc armature has been utilized.
For a plasma armature launcher, one of the dominating effects during a launch relates to the electrical resistance of the conducting plasma that conducts current between the two rails of a simple parallel rail launcher. The electrical resistance has three components: EQU R.sub.TOTAL =R.sub.bulk +R.sub.anode +R.sub.cathode ( 1)
where R.sub.anode and R.sub.cathode are the resistance associated with the plasma-to-rail interfaces and R.sub.bulk is the bulk resistance of the plasma. It is usual to express Equation 1 in terms of a voltage as: EQU V.sub.TOTAL =IR.sub.bulk +V.sub.A +V.sub.C ( 2)
where V.sub.A and V.sub.C are the anode and cathode voltage drops, respectively.
For a small bore electromagnetic launcher, calculations show that it is typical for the three contributions on the right side of Equation 2 to be of equal magnitude, typically 30 to 100 volts. However, as the bore of the launcher is scaled up to accept larger payloads, IR.sub.bulk increases approximately linearly with bore size, while V.sub.A and V.sub.C remain about constant. The relatively high plasma resistance can then lead to an undesirably high voltage and unnecessarily high energy loss. The present invention overcomes these drawbacks by replacing the bulk plasma with a better electrical conductor, which may be of solid or segmented construction. An arc or plasma at each end of this conductor serves to transfer current from the conductor to the adjacent conductive rails.
Thus, the metal conductor, which fills the bulk of the bore, serves to efficiently transfer current aross the bore and to transmit the generated force to the payload. The two plasmas primarily provide the electrical continuity between the metal conductor and the positive and negative rails and act as current transfer or voltage pickup elements.
An electromagnetic projectile launcher constructed in accordance with the present invention comprises a pair of generally parallel conductor rails, a source of high current, means for transferring current from the source of high current to the conductor rails, and an armature structure for conducting current between the conductor rails and for propelling a projectile along the rails. The armature structure includes a conductive element having a length less than the distance between the rails. A plasma at each end of the conductive element serves for conducting current between the conductive element and the rails. Copending commonly assigned application Ser. No. 381,603, filed on the same day as this application, discloses an electromagnetic projectile launcher employing a multiple current path combination armature and is hereby incorporated by reference.
The use of a plasma propelled armature has resulted in damage to the breech section of launcher rails during formation of the plasma. However, once the plasma is moving rapidly, very little, if any, damage occurs to the rails. An electromagnetic launcher may be constructed in accordance with this invention wherein metal-to-metal contact between the armature conductor and rails is maintained at low speeds, but an automatic transition to the previously described plasma/conductor armature would occur at high armature speeds.
The method of accelerating projectiles in such an electromagnetic projectile launcher comprises the steps of: supplying current to a pair of generally parallel conductive rails; conducting current between these rails by means of a conductor which is in physical contact with the rails and slidably disposed between the rails; and creating a plasma between each end of the conductor and the adjacent rail after the conductor has reached a predetermined velocity or position in the launcher barrel.